The present invention relates to a modified xcex8-Al2O3-supported nickel reforming catalyst and its use for producing synthesis gas from natural gas. More particularly, the present invention is to provide a nickel reforming catalyst expressed by the following formula 1, having improved coke resistance, high-temperature catalysis stability and catalytic activity, which is prepared by coating nickel or mixture of nickel and cocatalyst (M1xe2x80x94M2xe2x80x94Ni) on a xcex8-Al2O3 support modified with metal (M3xe2x80x94M4xe2x80x94ZrO2/xcex8-Al2O3), and its use for producing synthesis gas from natural gas through steam reforming, oxygen reforming or steam-oxygen reforming,
M1xe2x80x94M2xe2x80x94Ni/M3xe2x80x94M4xe2x80x94ZrO2/xcex8-Al2O3xe2x80x83xe2x80x83(1) 
wherein M1 is an alkali metal; each of M2 and M3 is an alkaline earth metal; and M4 is a IIIB element or a lanthanide.
Steam reforming, which produces a mixture (synthesis gas) of hydrogen and carbon monoxide from methane (natural gas), has become an important and fundamental chemical industrial process from long time ago. Synthesis gas produced by steam reforming of methane is widely used for producing methanol, hydrogen, ammonia, etc. Recently, it has been used for producing liquid fuel or oxygen-including compounds.
The biggest problem in the steam reforming is deactivation of reforming catalyst due to deposition of carbon. The carbon deposition can be calculated thermodynamically from molar ratios of hydrogen to carbon and oxygen to carbon in the product. Therefore, in steam reforming of methane, excess amount of steam is required to increase molar ratios of hydrogen to carbon and oxygen to carbon, in order to prevent deactivation of reforming catalyst. This promotes an aqueous gasification enough to obtain a synthesis gas with molar ratio of hydrogen to carbon monoxide larger than 3:1. Therefore, this process may be suitable for producing ammonia or high-concentration hydrogen. The current industrial steam reforming process of methane is being carried out under the condition of 730-860xc2x0 C. of a temperature, 20-40 atm of a pressure and 1:4-6 of methane-to-steam molar ratio.
And, for the industrial catalyst for the steam reforming, nickel catalyst is used D dominantly. A superior industrial reforming catalyst should have coke resistance and thermal and mechanical stability. For this purpose, selection of suitable support such as xcex1-alumina is very important.
The inventors suggested reforming reactions using modified zirconia-supported nickel catalyst [Korea Patent Application No. 99-50013; Korea Patent Application No. 2000-0054443; and Korea Patent Application No 2000-0057688]. Korea Patent Application No. 99-50013 discloses a modified zirconia-supported nickel catalyst and carbon dioxide reforming using it. Korea Patent Application No. 2000-0054443 discloses a cerium-modified zirconia-supported nickel catalyst and steam reforming using it. Further, Korea Patent Application No. 2000-0057688 discloses a modified zirconia-supported nickel catalyst and steam-oxygen reforming using it. The modified zirconia-supported nickel catalyst showed better activity and stability than those of conventional reforming catalysts when used in carbon dioxide reforming, steam reforming and steam-oxygen reforming even under severe condition (high space velocity and low steam-to-methane ratio) and at high temperature. Despite of good activity and stability, zirconia is not suitable to be used in reforming catalyst, because it is more expensive than alumina. Accordingly, it is required to use the more cost-effective xcex8-Al2O3 support and modify it with zirconia. Then, if it is coated with nickel catalyst, an inexpensive catalyst with good activity and stability can be obtained.
Inventors have developed a nickel reforming catalyst by coating xcex8-Al2O3 base support with zirconia modified with alkaline earth metal (M3) and IIIB element or lanthanide metal (M4), and coating nickel and alkali metal (M1) or alkaline earth metal (M2) cocatalyst. When this reforming catalyst was used in various reforming reactions of natural as, i.e., steam reforming, oxygen reforming or steam-oxygen reforming, it has showed equal or superior activity and stability compared with the modified zirconia-supported nickel catalyst we had suggested in Korea Patent Application No. 99-50013.
Accordingly, an object of the present invention is to provide a nickel reforming catalyst using xcex8-Al2O3 as base support, and its use for producing synthesis gas through reforming reaction.
The present invention is to provide a nickel reforming catalyst expressed by the following formula 1, which is used for producing synthesis gas mixture of carbon monoxide and hydrogen from natural gas,
M1xe2x80x94M2xe2x80x94Ni/M3xe2x80x94M4xe2x80x94ZrO2/xcex8-Al2O3xe2x80x83xe2x80x83(1) 
wherein M1, is an alkali metal; each of M2 and M3 is an alkaline earth metal; and M4 is a IIIB element or a lanthanide.
Synthesis gas mixture of carbon monoxide and hydrogen may be produced from methane in the presence of the nickel reforming catalyst by steam reforming, oxygen reforming or steam-oxygen reforming, under the condition of 0-6 of methane-to-steam molar ratio, 0-1 of methane-to-oxygen molar ratio, 600-1000xc2x0 C. of a reaction temperature, 0.5-20 atm of a reaction pressure and 1,000-1,000,000 cc/hrxc2x7g-cat of a space velocity.
Hereunder is given a more detailed description of the present invention. This invention relates to a reforming catalyst expressed by formula 1 and its use for producing synthesis gas by steam reforming, oxygen reforming or steam-oxygen reforming of natural gas.
The modified xcex8-Al2O3-supported nickel reforming catalyst according to the present invention is prepared by coating a modified zirconia (M3xe2x80x94M4xe2x80x94ZrO2) on xcex8-Al2O3 base support and coating nickel and cocatalyst (M1xe2x80x94M2xe2x80x94Ni) on it. Here, nickel is added preferably in the range of 3 to 20 wt. % based to xcex8-Al2O3. Each of M1 and M2 cocatalysts is added preferably in the range of 0 to 0.2 and 0 to 4 molar ratios based to nickel, respectively. Each of M3 and M4 is added preferably in the range of 0 to 1.0 and 0.01 to 1.0 molar ratios based to zirconium (Zr), respectively. Zirconia (ZrO2) is added preferably in the range of 0.01 to 1.0 molar ratio based to xcex8-Al2O3.
M1 is an alkali metal chosen from sodium, potassium and cesium; each of M2 and M3 is an alkaline earth metal chosen from calcium, magnesium and strontium, and M4 is a IIIB element chosen from yttrium, lanthanum and cerium, or lanthanide, wherein M3 and M4 are cocatalysts for enhancing coke resistance and catalytic activity.
The reforming catalyst according to the present invention may be prepared by coprecipitation, precipitation, sol-gel method, molten-salt or impregnation method. xcex8-Al2O3 is modified with zirconia to obtain support, coprecipitation or sol-gel method is preferable. When nickel and cocatalyst are coated on the support, molten-salt and impregnation methods are preferable.
Preparation of modified xcex8-Al2O3 support through coprecipitation is as follows. 1 M zirconyl chloride aqueous solution is purified by recrystallization from hydrochloric acid. 1 M nitrate solution of the metal to be modified on the xcex8-Al2O3 base support is added to this solution. After strongly stirring the mixture at 50-70xc2x0 C. for 4-8 hr, 29.8% ammonia water is added until pH reaches 10. The precipitate is filtered and washed with 5% ammonia water until no chloride ion is observed. It is further washed with distilled water. The filtered precipitate is placed in a drying oven and dried at 90-100xc2x0 C. for 10-12 hr. Then, it is put in a furnace and calcined in 700-900xc2x0 C. of air for 7-9 hr to obtain the modified xcex8-Al2O3 support.
Preparation of nickel- and cocatalyst-supported modified xcex8-Al2O3 support through impregnation is as follows. Modified (xcex8-Al2O3 support powder is added in distilled water and stirred to obtain a slurry. 1 M nitrate solution of nickel and the metal used as cocatalyst is added to this slurry. The mixture is stirred at 25xc2x0 C. for 5-7 hr. Then, the flask containing this mixture solution is connected to a rotary vacuum evaporator in order to evaporate water under pressure of 90-120 Torr and temperature of 70-90xc2x0 C. The obtained catalyst precursor powder is dried in a drying oven at 100xc2x0 C. for 12 hr and calcined at 400-500xc2x0 C. in air for about 4 hr to obtain the nickel reforming catalyst.
Preparation of nickel- and cocatalyst-supported modified xcex8-Al2O3 support through molten-salt method is as follows. The fact that nickel and cocatalyst metal precursor have low melting temperature is used. In order to obtain uniform and thin coating of metal component on the modified xcex8-Al2O3 support, the support powder and the metal precursor powder is finely ground and mixed well at room temperature. Then, the metal precursor component is decomposed through a well-defined heat treatment process to obtain the catalyst. The heat treatment process is as follows. All nitrates in the catalyst precursor powder are melt by heating it at 2xc2x0 C./min from room temperature to 400xc2x0 C. under argon flow of 100 mL/min in a furnace. The nitrates are decomposed at 400xc2x0 C. for 4 hr. Then, after changing argon flow to dry air flow, the temperature is increased to 650xc2x0 C. at 5xc2x0 C./min. The product is treated for 4 hr at 650xc2x0 C. The typical catalyst obtained from this method is Nixe2x80x94Ca/Cexe2x80x94ZrO2/xcex8-Al2O3 catalyst, which is prepared by using nitrates of nickel (melting point: 56.7xc2x0 C.) and calcium (melting point: 39.7xc2x0 C.) as metal precursor.
The above methods are only examples of preparing modified xcex8-Al2O3-supported nickel reforming catalysts according to the present invention. Other known preparing methods may be used if necessary.
Reaction condition of producing synthesis gas from natural gas using the nickel reforming catalyst according to the present invention is as follows
The nickel reforming catalyst expressed by formula 1 can be used in steam reforming, oxygen reforming or steam-oxygen reforming to prepare synthesis gas mixture of carbon monoxide and hydrogen from methane. In the reforming reactions, methane-to-steam molar ratio is preferred to be 0-6, and methane-to oxygen molar ratio is preferred to be 0-1. To be more specific, in the steam reforming, the methane-to-steam molar ratio is preferred to be 1-6; in the oxygen reforming, the methane-to-oxygen molar ratio is preferred to be 0.1-1; and in the steam-oxygen reforming, the methane-to-steam molar ratio is preferred to be 1-5 and the methane-to-oxygen molar ratio is preferred to be 0.1-1. The reaction temperature is 600-1000xc2x0 C. and the reaction pressure is 0.5-20 atm. The reaction gas is injected at 1,000-1,000,000 cc/hrxc2x7g-cat of space velocity.
As described above, the present invention provides a new industrial catalyst, which has equal or superior activity and stability compared to the expensive conventional modified zirconia-supported nickel reforming catalyst, by coating modified zirconia on xcex8-Al2O3 support and supporting nickel or mixture of nickel and cocatalyst on the support.
The following examples are intended to be illustrative of the present invention. However, they should not be construed as limiting the scope of this invention.